1. Field of the Invention
The present invention relates to apparatus for measuring oil and winding temperatures in a transformer and, more particularly, to using mechanical means to measure the oil temperature and electrical means to measure the winding temperature.
2. Description of Related Art
Power transformers are an integral and expensive part of the electrical power grid. Since they are such an expensive asset it is in the best interest of the utilities to maximize the length of service of their transformers and one way to do this is by monitoring and controlling the temperature of the transformers. The transformer insulating materials deteriorates with time and temperature at a rate according to an adaptation of the Arrhenius reaction theory so measuring the oil and winding temperatures of a transformer can be used to 1) control the rate of cooling, 2) give guidance on how to load the transformer, and 3) calculate the consumed life of the transformer.
Historically mechanical gauges have measured transformer oil temperatures using bourdon tubes, bellows or bi-metals and then calculated the winding temperatures by adding a heated well or element, to the oil temperature. Electrical gauges have measured transformer oil temperatures using RTDs or thermocouples and then calculating the winding temperatures by measuring the load current. The main advantage of a mechanical oil monitor is that it requires no electricity to operate. It can actuate cooling or alarm switches using only the heat generated from the oil in the transformer. It also has a long history in the industry and is the simplest and most familiar instrument to the greatest number of operations/maintenance personnel. The advantages of electrical monitors are ease of calibration, programmable, digital protocols, data logging, the ability to do calculations, measure winding temperatures and flexibility. We propose to combine the mechanical and electrical metering systems into one Hybrid device thereby maximizing the features of both and minimizing their limitations. By having the mechanical element measure the oil temperature and controlling mechanical switches the Hybrid monitor would continue to function even with loss of electrical power. The electrical portion of the Hybrid monitor would be able to provide all the features and advantages that come with an electronic device such as but not limited to digital protocols, data logging, measuring winding temperatures, ease of calibration, and the like.
In the power industry there are mechanical monitor/controllers that measure the oil temperature using bourdon tubes or bi-metals. Winding temperatures are simulated with heated wells or elements raising the temperature of the oil sensing element of the mechanical monitor/controller.
Electrical gauges measure transformer oil temperatures using RTDs or thermocouples. Winding temperatures are either simulated using loading currents to add to the oil temperature or measured directly using fiber optic probes.
Mechanical gauges have a very limited functionality of only being able to actuate switches for local alarming or controlling cooling.
Electrical gauges have an inherent issue with loss of supply power. Any loss of supply power means a complete loss of functionality. It is possible to back up electrical gauges with batteries or other circuitry but that raises the product cost and complexity. It would be advantageous to provide a transformer monitor having a mechanical sensor providing an oil temperature input to actuate mechanical switches and electrical switches.
It would also be advantageous to provide a transformer monitor that would function with loss of electrical power.
It would further be advantageous to have redundancy in a transformer monitor to measure oil temperature both mechanically and electrically.
It would further be advantageous to provide a transformer monitor that had the ability to measure winding temperature.
It would further be advantageous to provide a transformer temperature monitor that had local displays for both oil and winding temperatures.
It would further be advantageous to provide a transformer temperature monitor that had expanded capabilities such as data logging, winding simulation, loss of life calculations, etc.